The invention relates to an arrangement for decoding a digital video signal encoded as an MPEG elementary video stream. The invention also relates to television receivers, video recorders, and transmitters comprising such an arrangement.
An arrangement for decoding a digital video signal is disclosed in xe2x80x9cISO/IEC CD 13818: Information technologyxe2x80x94Generic coding of moving pictures and associated audio informationxe2x80x9d, Dec. 1, 1993, further referred to as the MPEG standard. Part 1 of this standard relates to the system aspects of digital transmission, Part 2 relates more particularly to video encoding.
MPEG2 is a packet-based time multiplex system. Data is transmitted in transport packets. Each transport packet contains data from exactly one elementary stream with which it is associated by means of its packet identifier. Examples of elementary streams are video streams, audio streams, and data streams. One or more elementary streams sharing the same time base make up a program. A typical program might consist of one video stream and one audio stream. One or more programs constitute a transport stream.
It is, inter alia, an object of the invention to provide an arrangement which renders it possible to implement new and known features in a more cost-effective manner.
According to the invention, the arrangement for decoding an MPEG elementary video stream is characterized in that the arrangement comprises means for decoding selected parts of said elementary stream, and means for rearranging said selected parts so as to constitute an ancillary video signal. As only selected parts of the elementary signal are decoded, the arrangement is considerably simpler and less expensive than a full-spec MPEG decoder.
As is known in the prior art, an MPEG encoded video signal includes autonomously encoded pictures (I-pictures) and predictively encoded pictures (P-pictures and B-pictures). The selected parts constituting the ancillary signal may be, for example, the I-pictures. In that case, the arrangement is simple because motion compensation circuitry and a large amount of memory can be dispensed with. An embodiment of the arrangement is characterized in that said selected parts are the DC coefficients of autonomously encoded pictures. Such an arrangement is extremely simple.
MPEG also allows parts of the signal to be scrambled, whereas other parts remain unscrambled. A further embodiment of the arrangement is characterized in that the selected parts are the unscrambled parts of the video signal.
The arrangement provides an ancillary video signal having a lower quality than the main signal from which it is derived. Various applications thereof are conceivable. A picture-in-picture television receiver, for example, may comprise the arrangement so as to obtain the ancillary signal for display as the picture-in-picture. In a multi-picture-in-picture television receiver, the arrangement may be used to decode a plurality of elementary video streams, and simultaneously display the respective ancillary signals as a mosaic picture. In a video recorder, the arrangement may be used to obtain a low-quality version of a video signal for separate recording so as to be reproduced at higher playback speeds. If the ancillary signal comprises the unscrambled parts of a scrambled main signal, it allows a video program to be viewed free of charge but at a low quality. The ancillary signal thus acts as an xe2x80x9cappetizerxe2x80x9d, attracting the viewer""s attention to the presence and contents of the main signal. When recorded simultaneously with the main signal on a digital video recorder, the ancillary signal also assists the user in finding the beginning of a scrambled program on tape.
The arrangement may also be used in transmitters. According to the invention, a transmitter for transmitting a digital video signal encoded as an MPEG elementary video stream, is characterized in that the transmitter comprises the arrangement for decoding said video signal, and means for transmitting the ancillary video signal as a further elementary video stream. The MPEG standard allows a program to comprise more than one elementary video stream. The ancillary video signal thus transmitted may serve the purposes mentioned before. For decoding the ancillary signal, a simple decoder is adequate. The transmitted ancillary signal requires only a low bitrate.